Electric field tailoring of thin film resistors



Nov. 19, 1968 F. J. PAKULSKI ET AL 3,411,203

ELECTRIC FIELD TAILORING OF THIN FILM RESISTORS Filed Jan. 14. 1966 K GAS 11 v SUPPLY T 15 13 Mme WREsTsTTvE FlLM \\\\\\\\\\\\QI/CERAMIC SUBSTRATE $$1 11 FIG 2 \\\\\=\\\\\\T A TRIM 3 0, 12%

20 ANNEAL 2.0 SHIFT L0 SILVER 1.0

% TRIM FIG. 3

7/ l M I INVENTORS FRANCIS J.PAKULSKI WILLIAM LWRIGHT ATTORNEY United States Patent 3,411,203 ELECTRIC FIELD TAILORING 0F THIN FILM RESISTORS Francis J. Pakulski, La Grangeville, and William L.

Wright, Poughkeepsie, N.Y., assignors to International Business Machines Corporation, Armonk, N.Y., a corporation of New York Filed Jan. 14, 1966, Ser. No. 520,761 9 Claims. (Cl. 29-620) ABSTRACT OF THE DISCLOSURE A method for down-tailoring the ohmic value of a resistor by an electric field irradiation with a jet of ionized gas particles.

This invention relates to a method and apparatus of tailoring resistors and more particularly to such method or apparatus wherein the tailoring is achieved by exposure of the resistive material to a jet of ionized particles.

In the field of microelectronic circuitry, resistors and capacitors are formed on a substrate by screen printing a resistive or dielectric material respectively with the appropriate electrodes being attached at the opposite surfaces of the layer. The area of the screened layer is made to be larger than required and the resistive or capacitive values is controlled by physically removing a portion of the material. One manner in which such material may be removed is by subjecting that portion to bombardment by particles of a fine grit or sand in a manner similar to sandblasting techniques. However, the respective microelectronic components are usually quite small having width and length dimensions of the order of a few millimeters and the use of such air abrasive techniques is such that control of the electrical characteristics can only be maintained within a rather wide range. Furthermore, such air abrasive techniques can result in damage to the substrate material and to adjacent components if they are not spaced sutficiently far apart.

Another manner in which a microelectronic resistor can be formed is to screen print a layer of resistive material such as aluminum oxide A1 0 and provide a carbon coating thereover such that an exposure of a portion thereof to an electron beam reduces the aluminum oxide on the surface to pure aluminum with the oxygen combining with carbon to form carbon dioxide, the re duction being initiated by the heat from the electron beam. Of course, the heat generated in this manner can have adverse effects upon the circuit substrate as well as terminal connections and furthermore the electron bombardment can cause undesirable physical defects.

Current interest in resistive materials is primarily directed toward such materials that might be described as semiconductors because of the particular mechanism of electrical energy flow. A particular material of this type is palladium oxide (PdO) to which has been added a conductive material such as silver or gold. To provide structural strength for a layer of such material, the materials are mixed with a glass binder in a grinding operation such that a subsequent screen printing and firing allow the glass to flow sufliciently throughout the composition. A layer of such a resistive material has the appearance of a glaze and the resistors are referred to as glaze resistors. Another method of providing a protective coating is to first screen and fire a layer of the resistive material after which a layer of a glass is screen fired thereover. In either case the purpose of the glaze is to prevent exposure of the resistive material to moisture or other vapors that might effect the electrical properties thereof and it Will be appreciated that tailoring of the resistive material by any of the methods discussed above will damage the proice tective coating defeating its purpose and destroying its utility.

It is then an object of the present invention to provide an improved apparatus for tailoring or trimming of glaze resistors.

It is another object of the present invention to provide improved apparatus for the tailoring of glaze resistors which does not damage a protective coating provided for such thin film resistors.

It is still another object of the present invention to provide an improved method for tailoring a glaze resistor.

The effect of radiation upon the electrical properties of the materials have been observed by many investigators. The manner in which such radiation effects either the number or the mobility of the current carriers has been fairly Well described depending upon the type of radiation involved. When the radiation is in the form of energized particles, the change in electrical properties is primarily due to the introduction of defects into the material. When the material is metallic the effect of fast particle radiation is one of increasing the residual resistivity and the number of conduction electrons is not readily altered to any extent. On the other hand, in semiconductor materials the number of conduction electrons is quite small and large changes in electrical properties can occur upon radiation by fast particles causing the introduction of defects into the material structure. When the radiation is of an electromagnetic nature such as gamma ray or X-ray radiation the effect upon the material is primarily to increase the number of conduction carriers which effect may be explained as one of a variety of phenomenon such as the photoelectric effect, the Compton effect or pair production depending upon the energy of the radiation.

It has been discovered that the use of lower frequency high intensity fields results in reorientation of conductive material within the resistive matrix. This reorientation usually results in the means increase of the conductivity of a resistive material and a microelectronic resistor may be made of a smaller dimension and the resistance may be tailored downwardly by exposure to radiation to thereby increase conductivity. The radiation need not be of any particular frequency and it has been discovered that exposure to the field associated with a plasma jet can affect the desired tailoring of a resistive material. Furthermore, with such means of tailoring, there is no harmful effect to a glaze or other type of protective coating placed over the resistive layer. Means by which such a plasma jet of ionized gas may be created may include apparatus for passing an ionizable gas such as air, oxygen or nitrogen through a high voltage alternating current are with energy of the are being controlled by controlling the current passing through the arc. Furthermore, appropriate circuitry can be provided to test the resistance of the resistive layer during the tailoring process such that the resistance tailoring may be precisely controlled by controlling the energy of the arc.

A feature, then, of the present invention resides in a method of tailoring a layer of resistive material by exposure of a portion thereof to the field-s associated with a plasma jet or jets of ionized gas.

More specifically, a feature of the present invention resides in means for tailoring a resistive layer of material whereby a jet of ionizable gas is applied through a high voltage alternating current are such that the degree of tailoring is controlled by controlling the energy supplied to the arc.

These and other objects, advantages and features will become more readily apparent from the review of the following specification when taken in conjunction with the drawings wherein:

FIGURE l is a cross sectional view of apparatus as contemplated for employment with the present invention;

FIGURES 2A and B are a set of curves representing resistance change as achieved by the present invention; and

FIGURE 3 is a waveform of the electrical noise associated with the plasma jet employed in the present invention.-

The process of the present invention comprises the trimming or tailoring of the resistor by exposure to a plasma jet and subsequently annealing the resistor. This process utilizes the electric field sensitivity phenomena exhibited by the silver and similar metals in a glazed resistor film, that is, a resistor of an oxide semiconductor material with a noble metal additive. As opposed to air abrasive trimming, plasma jet trimming reduces the resistance of a resistor. This down-tailoring is brought about by bringing the tip of the plasma jet into contact with the surface of the resistor. The resistance of the resistor is reduced to a desired value by scanning the surface of the resistor with the plasma jet. No detectable amounts of material are removed from the resistor during this process.

This plasma jet or jet of ionized gas is generated by using a high voltage AC are as an energy source and an air jet asa gas supply although other ionizable gases may be employed. Associated with the plasma jet is the [generation of a wide spectrum of electrical noise, a major component of which occurs at a frequency of about kc.

Thermal pre-aging or annealing of the tailored resistors was found to improve the stability of the resistors. Such annealing cycle may consist of storage in air at 200 C. for a period of approximately 15 to hours. However, other pre-aging cycles may be used.

Materials of the type for which the present invention is contemplated for use are disclosed in patent application, Ser. No. 313,032, filed Oct. 1, 1963, now Patent No. 3,248,345 by Arthur H. Mones and Kenneth E. Neisser and assigned to the assignee of the present invention. Particularly, such a material is a metal-glaze composition having a conductive element selected to be a p-type semiconductor oxide material which is doped with elements of particular valence to adjust the resistivity thereof. For printed type resistors, such a material is fixed and screened with an insulative glass material for reasons described above, the particular method of forming such resistors being described in the above referred to copending application. A specific oxide of the type described is palladium oxide having a small percentage of silver although other oxides and noble metals may be employed.

.FIGURE 1 illustrates an application of the present invention whereby a resistive film of the type described above is exposed to a jet of ionized gas to achieve downtailoring, that is resistance reduction, prior to an anneal cycle. The manner in which the ionized gas or plasma jet is created and the characteristics thereof will be more thoroughly discussed below. More specifically, FIGURE 1 discloses diagrammatically the composition of the resistive film which has been screened onto alumina substrate and fired for approximately 30 minutes at about 750 C. in an oxidizing atomsphere. Analysis of this glaze resistor indicates the presence of two phases, namely palladium oxide and a silver-palladium alloy as well as the glass phase which serves as a binder and protective coating. During the firing of the initial paste, no oxidation occurs below 330 C. Between 330 C. and 520 C., palladium oxidizes to PdO and at about 520 C. the silver and palladium start to alloy. As the temperature is increased above 520 C., the reaction is progressively shifted to the right and an equilibrium is obtained.

Conduction in the PdO-PdAg glaze resistor is known to be of the P-type and PdO which provides the major portion of resistance, is an oxide semiconductor with a metal deficit. Since the semiconducting properties of PdO are due to departure from true chemical stoichiometry, changes in stoichiometry of PdO would be expected to result in overall change in the resistivity of the glaze resistor. For increase 'in resistivity, these changes in stoichiometry would consist in removal of cation vacancies either by diffusion of Pd or other metal into the PdO lattice or removal of oxygen from the PdO lattice. In either case, a more stoichiometric condition and a higher resistivity would be anticipated for the PdO. Conversely, decreases in resistivity as a result of the creation of cation vacancies would be expected from the removal of palladium or silver which exist to a certain extent in the PdO lattice. Down-tailoring by exposure to a jet of ionized gas according to the present invention, is directly related to the percentage of silver in the initial paste as illustrated in FIGURE 2A which illustrates the maximum decrease in resistance that can be obtained as a function of the percentage of silver. As indicated in FIGURE 2A, when there is only 6 percent silver in the initial paste, the maximum obtainable down-tailoring is no more than 30 percent. When there is 12 percent silver in the resistant paste, 50 percent decrease in resistance can be obtained and when there is 15 percent silver in the resistant paste, 60 to 70 percent reduction of resistance can be obtained.

As indicated in FIGURE 2B, the percentage shift of the resistive values, during the subsequent anneal heat treatment, decreases with an increased percentage of silver in the initial resistive paste and increases with an increased percentage of trim during the previous tailoring process. It should be noted that the abcissa in FIGURE 2B is representative of the percentage of maximum trim obtainable. Thus, for 6 percent silver in the initial paste, the shift of the trimmed resistive values will vary by as much as 5 percent for up to 50 percent of maximum trim, maximum trim for 6 percent initial silver being approximately 30 percent of the original resistive value as seen from FIGURE 2A. For 12 percent silver in the initial paste, the shift during anneal of the trimmed resistive values will vary by as much as 3 percent for maximum obtainable trim. For 15 percent silver in the initial resistive paste, the shift during anneal of the trimmed resistive value will vary from a 0-.5 percent to a -0.25 percent or approximately 30 percent of maximum trim and then change to a variation of a 1 percent for maximum trim.

Referring again to FIGURE 1, the apparatus for forming the plasma jet will now be described. In FIGURE 1, nozzle 10 is formed of a block of insulating material which may be well any known electrical insulation. Conduit 11 is provided along the axis of the block to provide a supply of ionizable gas through aperture .12 and two probes 13' are inserted through the sidewalls of the block and positioned with their ends at aperture 12 for the purpose of establishing an arc across the aperture. On use of a suitable ionizing gas, the plasma tip 14 is formed and its size may be adjusted by adjustment of gas flow.

' FIGURE 3 represents a waveform obtained by placing a probe in the form of a single copper wire in the proximity of plasma tip 14 and illustrates the electric noise generated thereby when the gas employed is air. Power generator 15 was adjusted to provide a 3000 volt 6O cycle output across the probes in series with resistor 16 which was selected to be 83 ohms to provide the proper current limiting for the circuit. The periodicity of the waveform in FIGURE 3 represents the 60 cycle voltage applied to the probes, the entire waveform of FIGURE 3 representing one cycle thereof. It will be appreciated that the electric noise generated by the plasma during each half of the 60 cycle signal contains a wide spectrum of frequencies up to the megacycle range with a predominant frequency of 15 kc. Attempts to employ nitrogen and forming gas H 90% N produced little or no plasma jet or electric noise.

Exemplary values of actual resistances obtained for resistor formed of the resistive material described above and trimmed according to the present invention, are listed below where the resistive film is approximately 70 mils wide and 100 mils long, the resistance being measured across the electrodes between which the film resides.

Percent; Percent Percent: Ag Ra Rt Ra Trim Anneal Shift.

Where R is the measured resistance in ohms before trimming, R, is the measured resistance in ohms after trimming and R is the measured resistance in ohms after anneal.

It has already been described that overtrimming or exposure to the plasma jet can result in a burning or physical damage to the resistive film. Therefore, the film should not be statically exposed to the plasma jet which should be scanned or passed over the surface of the resistor. To achieve optimum ionization of the gas employed, it was found that the electrodes in FIGURE 1 should be spaced no more than mils apart. For a plasma jet of approximately a 20 mil width, it was found that the jet could be scanned along 100 mils of a resistive surface in a time period of 6 seconds without causing any undue damage to the resistive film. To achieve maximum trimming, a number of scans could be made until the desired resistance was obtained. Of course, faster scanning rates could be employed with a greater number of scans needed to achieve maximum trimming.

In addition to the preferred combination of palladium oxide and silver, other oxides such as platinum oxide (PtO), and rhodium oxide (RhO) may be employed with the conductive metal additive being gold or platinum as well as silver. In the case of these oxides as well as with palladium oxide, it will be appreciated that the increased amount of the conductive metal in the initial paste results in greater trimability as well as in greater stability in the annealed product. However, it will be appreciated, as may be inferred from the above table, that an increased amount of conductive metal in the initial paste requires a larger size resistor in order to achieve large resistive values.

As employed herein the percentages stated are stated as percentages by weight unless otherwise specifically indicated.

While the invention has been particularly disclosed and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes and modifications may be made therein without departing from the scope and spirit of the invention as claimed.

What is claimed is:

1. A method of forming a glaze resistor having a particular resistive value which method comprises:

forming a resistor of an oxide semiconductor material with a noble metal additive; and

subjecting a portion of said resistor to an electric field radiation to reduce the resistance thereof.

2. A method according to claim .1 wherein the electric field irradiation is by exposing a surface of said resistor to a jet of ionized gas particles.

3. A method according to claim 1 where the resistor is formed of an oxide selected from the group consisting of palladium oxide, platinum oxide and rhodium oxide and the noble metal is selected from the group consisting of silver, gold and platinum.

4. A method according to claim 2 where the gas particles are oxygen and nitrogen.

5. A method of forming a resistor having a particular resistive value which method comprises:

forming a resistor of an oxide semiconductor material with a noble metal additive;

exposing the surface of said material to an electric field formed by a jet of ionized gas particles to reduce the resistance thereof; and

subjecting the resistor thus irradiated to an environment having an elevated temperature to stabilize the resistive value thereof.

6. A method according to claim 5 wherein said electric field formed by a jet of ionized particles is created by passing said particles through a high voltage alternating current are.

7. A method according to claim 5 wherein the resistor is formed of an oxide selected from the group consisting of palladium oxide, platinum oxide and rhodium oxide and the noble metal is selected from the group consisting of silver, gold and platinum.

8. A method according to claim 7 wherein the percentage of the noble metal added to the semiconductor material is selected according to the maximum resistance reduction required.

9. A method according to claim 6 wherein the gas particles are oxygen and nitrogen and upon ionization are characterized by an electric field radiation of approximately 15 kc.

References Cited UNITED STATES PATENTS 2,945,119 '7/1960 Blackman 21912l 2,994,847 8/1961 Vodar.

3,108,019 10/1963 Davis.

3,252,831 5/1966 Ragan.

3,261,082 7/1966 Maissel et al. 29620 3,300,561 1/1967 Foex 219121 3,308,528 3/1967 Bullard et al. 29620 OTHER REFERENCES E. R. Dean: Effect of Thermal Aging on the Electrical Resistivity of Thin Alloy Films, J. of Applied Physics, v. 35, No. 10, October 1964, pp. 29302933.

JOHN F. CAMPBELL, Primary Examiner.

I. CLINE, Assistant Examiner. 

